If I activate my iPhone with T-Mobile, can I still update it?

if I unlock it, will I still have the simplicity of updating it to Apple's or AT&T's future applications?

If you unlock your iPhone 3g from carrier lock, you can able to switch to other GSM sim cards internationally . To unlock it, use Remote unlocking service . You can get it from any of the online vendors like Unlock-Zone.com . After unlocking your iPhone, you can update it . It wont gets locked again . Because, it is one of the permanent unlocking method . After unlocking it, you can use AT&T applications with it .

will I be able to lock it back into place if a change of mind comes to plan to become an AT&T member.

Once, your phone gets unlocked , it open to all networks . No need to lock it again .



AND one more thing, is there a prepaid plan for the iPhone?

Yes it is a prepaid plan for your iPhone .